Resin films are used in various fields, for example, for liquid crystal display devices, in view of their chemical characteristics, mechanical characteristics, electrical characteristics, and the like. Specifically, various resin films, for example, transparent protective films for protecting polarizing elements on a polarizing plate are arranged in an image display area of a liquid crystal display device. As such resin films, for example, cellulose ester films which are excellent in transparency are widely used.
A resin film such as a cellulose ester film can be produced using a resin solution (dope) in which a material resin such as a cellulose ester resin is dissolved into a solvent. Specifically, a method for producing a resin film using such dope includes, for example, a solution cast film-forming method. The solution cast film-forming method is used for producing an elongate resin film by casting dope on a running support to form a cast membrane, drying the cast membrane to the degree that the cast membrane can be released, releasing the cast membrane from the support to obtain a film, and conveying the released film by a conveying roller, while performing a drying operation, a stretching operation, and the like on the film.
When a resin film is produced by the solution cast film-forming method as above, membranes derived from a dope, i.e. so-called membrane burrs, may be formed at the outer surface around a discharge port of a casting die, specifically at the outer surface around the central portion of the discharge port of the casting die or at the outer surface around both ends of the discharge port of the casting die. Such membranes may gradually grow with the progress of production of resin films. There has been a problem such that such membranes may disturb the flow of the casting dope to block the production of resin films. There has also been a problem such that the membranes detached from the casting die may damage the formed resin film.
A method may be mentioned that uses a casting die having a dope-contacting surface which has been subjected to a surface treatment in order to suppress the formation of the membrane, particularly the formation of the membranes at the outer surface around the central portion of the discharge port of the casting die during production of resin films by the solution cast film-forming method. Specifically, the methods described in Patent Documents 1 and 2 may be mentioned.
Patent Document 1 discloses a method for producing a propylene film by extruding a propylene resin through a T-die in which the T-die used has a ceramic thermal spray coated part formed by thermal spray coating of a ceramic material at the edge of a lip opening.
Patent Document 1 discloses that the propylene film obtained does not contain or is not contaminated with a foreign material attached at a lip part surrounding the lip opening of the die.
Patent Document 2 discloses a method for producing a cellulose resin film by melt film formation comprising discharging, as a sheet shape, a molten resin melted in an extruder from a die onto a running or rotating cooling support to cool and solidify the same, wherein a contacting part of the die which is brought into contact with the molten resin has a surface roughness of 0.3 μm or less, a contact angle of 50° or more and a surface energy of 60 mN/m or less.
Patent Document 2 discloses that generation of streak troubles which cause deterioration of surface quality of the film upon discharge of the molten resin from the die can be prevented and the cellulose resin films having excellent optical properties can be efficiently produced.
In order to suppress the formation of the membranes, particularly the formation of the membranes at the outer surface around both ends of the discharge port of the casting die, a method may be mentioned in which, when a dope is discharged from the casting die so as to cast the dope onto a running support to form a cast membrane, a solvent capable of dissolving a transparent resin contained in the dope is allowed to fall from longitudinally opposite ends of the discharge port of the casting die at the same time as discharge of the dope. Specifically, the method described in Patent Document 3 may be mentioned.
Patent Document 3 discloses a solution film formation method comprising discharging a dope containing a polymer and a solvent from a discharge part of a casting die onto a support running endlessly to form a casting bead between the discharge part of the casting die and the support and to form a cast membrane on the support, releasing the cast membrane from the support as a wet film and drying the released wet film to obtain a film, wherein an anticoagulant liquid is discharged from a liquid supplying nozzle having a nozzle port above the discharge part towards the vicinity of the end of the discharge part, the anticoagulant liquid discharged from the liquid supplying nozzle is guided by means of a guide path to the vicinity of the end of the discharge part, a liquid pool of the anticoagulant liquid is formed in the vicinity of the end of the discharge part, thereby supplying the anticoagulant liquid to the end of the discharging part and to an end along the width direction of the casting bead.
Patent Document 3 discloses that the anticoagulant liquid can be stably and accurately supplied to both ends of the casting bead.    Patent Document 1: Japanese Patent Application Laid-open No. 2006-289888    Patent Document 2: Japanese Patent Application Laid-open No. 2008-194956    Patent Document 3: Japanese Patent Application Laid-open No. 2009-45928